The copy number of the Saccharomyces cerevisiae mitochondrial genome varies.
Two major influences on mitochondrial DNA (mtDNA) copy number are genetic
background and carbon source. While genes required for maintenance of mtDNA have
been thoroughly investigated, genes that increase mtDNA copy number have not
been systematically identified. We report a screen to determine mtDNA copy
number in the yeast haploid deletion collection, focusing subsequent work on
strains with elevated mtDNA copy number. Deletion strains were grown on glucose
and on glycerol, and total DNA was quantitatively isolated. Nuclear DNA and
mtDNA copy numbers were quantified by multiplex quantitative PCR. Several
classes of genes were identified with increased mtDNA copy number. Nine members
of the glucose sensing pathway had high mtDNA copy number, as predicted for
cells defective in glucose-mediated repression of mitochondrial biogenesis.
Another class of genes slows the cell cycle at various points, producing larger
yeast with a larger mitochondrial volume. A third class consists of deletions of
mitochondrial ribosomal proteins. These mutations may compromise mitochondrial
function, inducing mitochondrial proliferation. Other functional classes of
genes were also identified. We are characterizing growth rate, cell size,
mitochondrial volume, and mitochondrial membrane potential in the strains with
significantly altered mtDNA copy number to better understand the pathways that
promote mtDNA biogenesis.

Regulation of the Mitochondrial Compartment in S. cerevisiae

Mitochondrial proliferation (mitochondria stained with
red with Giemsa Trichome stain) observed in muscle fibers from a patient
with a mitochondrial DNA mutation. A number of different mutations that
compromise oxidative phosphorylation can trigger an increase in the
mitochondrial compartment. Image courtesy of Washington University.

Mitochondrial function has been genetically linked to aging through screens
in the the yeast Saccharomyces cerevisiae, the worm C. elegans, and the fruit
fly Drosophila melanogaster. Mutations that alter the efficiency of respiration
can dramatically increase lifespan in many organisms. The number of mitochondria
per cell is affected by the conditions under which cells are grown and the
metabolic state of the cell. Although many mutations are known that result in
the loss of mitochondrial DNA, little is known about the pathways that
upregulate the number of mitochondria per cell or the copy number of the
mitochondrial genome.